The present invention relates to methods and apparatus for transmitting compressed digital image signals, such as digital image signals subjected to discrete cosine transformation (DCT) for highly efficient coding.
Digital video tape recorders (VTRs) are available for recording digital video signals on magnetic tape with the use of rotating heads. Digital video signals contain large amounts of data, so that it is desirable to employ a highly efficient coding technique for compressing such data for transmission, for example, for recording on magnetic tape. Discrete cosine transformation has been employed for this purpose.
Discrete cosine transformation carries out a transformation of image data into the frequency domain. An image provided as a frame of digital data is subdivided into blocks of data, for example, eight-by-eight pixel blocks, and each of the blocks is transformed into the frequency domain to effect an orthogonal transformation thereof. The DCT process yields blocks of frequency coefficient data in eight-by-eight format. Thereafter, the blocks of coefficient data are variable length encoded utilizing for example, run length encoding, Huffman encoding, or the like, prior to transmission. Typically, a block synchronization signal is added to the variable length encoded data at intervals of predetermined data amounts for transmission.
Digital video recording apparatus, such as digital VTRs and apparatus for recording on disc-shaped recording media, normally record each field or frame of video data entirely in a respective track or in a respective group of tracks. However, when the video data is variable length encoded, the amount of data varies from frame to frame due to variations in the informational content of the various frames. In order to record the data of each field or frame entirely in a respective track or group of tracks, it is necessary to ensure that the amount of data within each field or frame is limited so that it does not exceed a predetermined target value. For this purpose, a buffering process is carried out to control the amount of data generated for each field or frame interval. However, since the amount of data in a given field or frame is very large, both a large memory capacity and complex processing circuitry are required for this purpose.
Consequently, it is desirable to employ a buffering process which controls data amounts in predetermined data intervals which are considerably shorter than one frame. Such data intervals are referred to as "buffering units" and, as ultimately included in sync block format, as "video groups".
Each buffering unit or video group has a predetermined quantization interval for the data included therein. Often, data from different video groups are contained in the same sync block, so that respective quantization numbers representing the quantization intervals of each of the video groups should also be included in each sync block. To decrease redundancy in the data, however, only one quantization number is transmitted for each video group. Consequently, when the quantization number for a respective video group suffers an error in transmission, none of the data in the corresponding video group can be recovered.
A further problem occurs when the data is reproduced in a variable speed reproduction mode, that is, wherein the tape speed differs from that at which the data was recorded. In the variable speed reproduction mode, the scanning path of the reproducing heads does not correspond with the tracks on the tape, so that the data is not reproduced continuously. Consequently, in this mode the validity of each reproduced sync block must be determined. When the data of a video group exceeds the capacity of a sync block, the quantization number for that video group may be included in one sync block while a portion of the corresponding data may be included in one or more other sync blocks. Accordingly, an otherwise valid sync block may be reproduced in the variable speed reproduction mode, while its quantization number may be included in another sync block which was not reproduced. In that event, none of the data within the corresponding video group can be recovered. This leads generally to a overall decrease in the amount of data that can be reproduced in the variable speed reproduction mode.
If, however, the amount of data in each video group is smaller than the capacity of each sync block, the conventional sync block format may be illustrated as in FIG. 1, in which it is seen that the number of video groups in the various sync blocks varies. As illustrated in FIG. 1, each sync block includes first a block synchronization signal SYNC, then additional information AIN0, AIN1 and AIN2 each including a quantization number which identifies the quantization interval used in the buffering process for a respective video group which is contained in that sync block. Following the additional information, variable length encoded DCT coefficient data of either two or three video groups are included. Finally, parity data PT, generated as a result of error correction encoding, are included. As seen in FIG. 1, data from up to three video groups may be included in a single sync block. Consequently, each sync block includes at least three items of additional information data, that is, AIN0, AIN1 and AIN2 as shown in FIG. 1. The result is that a considerable amount of redundant information is included.
A further disadvantage of the conventional sync block format is illustrated in FIG. 2 wherein a single sync block includes data from three separate video groups. It will be seen that due to the arrangement of the data in this fashion, a number of "empty" areas, designated "a", "b" and "c" in FIG. 2, result That is, because the data is variable length encoded, each group is likely to include less than a whole number of bytes, resulting in the empty area "a" at the end of the video groups as shown in FIG. 2. Moreover, the area "b" is provided at the beginning of the first, partial video group as a delimiter of the variable length encoded symbols continued from the previous sync block, while the area "c" is a delimiter of the variable length encoded symbols of the third video group which is continued into the following sync block.
As noted above, in the variable speed reproduction mode, data is not reproduced continuously so that the validity of each sync block must be determined. It is important, therefore, that quantization numbers for each video group present in each sync block be included therein. Consequently, as shown in FIGS. 1 and 2, both unnecessary data and meaningless data are contained in each sync block, so that the capacity to convey information is lessened and the amount of data which ultimately can be reproduced is decreased. It would be desirable, therefore, to include only a single quantization number in each sync block, as well as to decrease the number of video groups contained in each sync block.